1. Field of the Invention
The first invention relates to a method for processing with hydroxide ions in ultra-pure water, and more particularly relates to a processing method with which a workpiece can be subjected to removal processing or to oxide film formation processing by using only ultra-pure water and increasing the ion product is to provide a thereof. The second invention relates to a method for washing with hydroxide ions in ultra-pure water, with which hydroxide ions are supplied near the surface of a material to be washed, and particularly a semiconductor wafer or the like, and a high-speed shear flow of ultra-pure water is created, which makes it possible to completely remove any fine impurity metals adhering to the surface of the material being washed.
2. Description of the Related Art
Progress in scientific technology in recent years has led to new materials being developed one after the other, but an effective means of processing these new materials has yet to be established, with such technology always seeming to lag behind the development of new materials.
Recently, as the elements that make up all kinds of devices have become smaller and more precise, and as manufacturing on the sub-micron level has become more commonplace, the processing method itself increasingly has an effect on the characteristics of the material. In this situation, with a processing method in which part of the workpiece is physically and destructively removed with a tool, as with conventional mechanical processing, this processing can give rise to numerous defects in the workpiece, as a result of which the characteristics of the workpiece may suffer. A matter of great importance is therefore how to perform such processing without sacrificing the characteristics of the material.
Chemical polishing, electrolytic working, and electrolytic polishing are some of the special processing methods that have been developed as a means for solving the above problem. Unlike with conventional mechanical processing, these processing methods involve removing material by bringing about a chemical elution reaction. Therefore, defects such as work modified layers or dislocation due to plastic deformation do not occur, allowing processing to be carried out without the characteristics of the material being sacrificed as mentioned above.
A type of processing method that has been attracting even more attention involves the use of chemical interaction between atoms. This makes use of microparticles, radicals and so forth with high chemical reactivity. These processing methods remove material through a chemical reaction with the workpiece on the atomic order, so the processing can be controlled on the atomic order. Examples of these processing methods developed by the inventor include EEM (Elastic Emission Machining; Japanese Laid-Open Patent Application H1-236939) and plasma CVM (Chemical Vaporization Machining; Japanese Laid-Open Patent Application H1-125829). EEM makes use of a chemical reaction between microparticles and the workpiece, and therefore affords processing on the atomic order without sacrificing the characteristics of the material. Plasma CVM makes use of a radical reaction between the workpiece and radicals produced in a plasma at atmospheric pressure, and also affords processing on the atomic order.
With the above-mentioned electrolytic processing or electrolytic polishing, it has been conventionally held that the processing proceeds through electrochemical interaction between the workpiece and the electrolyte (an aqueous solution of NaCl, NaNO3, HF, HCl, HNO3, NaOH, or the like). Also, as long as electrolyte is used, the workpiece will inevitably be contaminated by the electrolyte.
In view of this, the inventor thought that hydroxide ions (OH ) might be acting on the processing in neutral and alkaline electrolyte, and reached the conclusion that if this were true then processing would even be possible with water in which hydroxide ions were present in even a tiny amount. The possibility of such processing was also confirmed experimentally, and in Japanese Laid-Open Patent Application H10-58236 the inventor proposed a processing method in which only ultra-pure water from which as much of the trace impurities as possible have been removed is used, this is subjected to a ion product increasing treatment for increasing the hydroxide ion, a workpiece is immersed in this ultra-pure water with elevated hydroxide ion concentration, and removal processing or oxide film formation processing is performed through a chemical elution reaction or oxidation reaction involving these hydroxide ions. In addition, the inventor proposed to utilize an electrochemical interaction on a solid surface having a function of an ion exchange or a catalyst as a hydroxide ion increasing treatment. This created a novel processing method with which the hydroxide ions in ultra-pure water can be utilized to perform clean processing, without any impurities being left behind on the processing surface. This processing method is expected to find application in a very wide range of fields, including the manufacture of semiconductors.
Nevertheless, it is a known fact that the hydroxide ion concentration is extremely low in ultra-pure water, being only about 10xe2x88x927 mol/L at 25xc2x0 C. and 1 atm, and even if the hydroxide ion density is increased with an ion exchange film, the increase is still only by a factor of about 103 to 104 at most. This represents an ion density of 1/104 to 1/103 for 1 mol/L (1N) NaOH, and the processing rate is still too low for practical processing purposes.
Meanwhile, washing methods for removing fine impurity metals that adhere to the surface of the material being washed include chemical washing and physical washing. In particular, since electronic circuits are formed on the surface of a semiconductor wafer in a fine pattern on the sub-micron order, any metal contaminants on the surface thereof can have a tremendous effect on the performance of the device, and also lower the yield and hamper cost reduction efforts. Accordingly, a variety of washing methods have been proposed and put to practical use.
Typical examples of chemical washing include washing with an acid or hydrogen fluoride, and fluorocarbon washing, which has become problematic because it leads to depletion of the ozone layer. Typical examples of physical washing include ultrasonic washing in ultra-pure water, and a method in which adhering microparticles are contracted and expanded by cooling or heating the washed material so as to strip them from the surface of the material.
It is not easy, however, to remove impurity metals or impurity metals in an ionic state firmly adhering through interaction (a type of chemical bonding) at the interface of the washed material, without damaging the surface of the material, and conventional washing methods cannot be considered effective. Specifically, the surface of the washed material is corroded by the washing liquid with chemical washing, while physical washing scratches the surface of the washed material. Also, once foreign matter has been removed from the surface of the washed material, it can re-adhere to the same surface, which is extremely difficult to avoid.
In order to eliminate impurity metals adhering through chemical bonding to the surface of a washed material, the inventor predicted theoretically and confirmed through experimentation that a shear flow of at least a specific strength is required on the washed material surface, that is, that the shear flow must have at least a specific velocity gradient, and discovered that it is effective to concurrently use a chemical elution reaction between impurity metals and hydroxide ions in ultra-pure water. As mentioned above, though, the hydroxide ion concentration in ultra-pure water is extremely low, so low that a practical washing efficiency cannot be achieved.
In this prior art situation, the inventor solved all of these problems and perfected the present invention upon recognizing that the removal processing of a material and washing to remove substances firmly bonded or adhering to a material surface are both the same problem.
Specifically, the first object of the present invention is to provide a truly ideal and practical method for processing with hydroxide ions in ultra-pure water, in which the hydroxide ion density is further increased at the processing surface of a workpiece and any atoms of the workpiece that have bonded with hydroxide ions are quickly removed from the processing surface, which raises the processing rate and allows clean processing to be performed, without any impurities being left behind on the processing surface of the workpiece, using the hydroxide ions in ultra-pure water.
In view of this, to achieve the stated first object, the first invention is a method for processing with hydroxide ions in ultra-pure water, comprising the steps of disposing a workpiece and a high pressure nozzle a specific distance apart from each other inside a washing tank containing only ultra-pure water, providing an ion exchange material or catalyst material that increases the amount of hydroxide ions between the processing surface of said workpiece and the distal end of the high pressure nozzle facing said surface;
applying a voltage using the high pressure nozzle as the cathode and the workpiece as the anode, generating a high-speed shear flow of ultra-pure water sprayed from the high pressure nozzle near the surface of the workpiece, supplying hydroxide ions produced from the ultra-pure water to the workpiece surface, and subjecting the workpiece to removal processing or to oxide film formation processing by means of a chemical elution reaction or oxidation reaction brought about by the hydroxide ions.
If the spray opening in the high pressure nozzle is a round hole here, then spot processing can be performed, allowing the workpiece surface to be processed in any form desired, and if the spray opening in the high pressure nozzle is a slit, line processing can be performed, allowing the workpiece surface to be processed into a flat shape, undulating shape, or cylindrical shape over a large surface area. Also, it is preferable if a recovery means is provided on the downstream side of the high-speed shear flow generated by the high pressure nozzle, and any process reaction substances removed from the workpiece are recovered, because the removed processing reaction substances can be prevented from adhering to the workpiece surface.
With a method for processing with hydroxide ions in ultra-pure water as in the first invention described above, hydroxide ions are produced by an ion exchange material, and these are efficiently supplied to the workpiece surface by an electrical field and the flow of ultra-pure water sprayed from the high pressure nozzle, the result of which is that the density of hydroxide ions is higher near the workpiece surface. Also, any processing reaction substances produced in the reaction between the hydroxide ions and the workpiece atoms are immediately removed from this surface by a high-speed shear flow of ultra-pure water, so that a fresh processing surface is always exposed, and this markedly speeds up the processing. Furthermore, the processing apparatus can be more compact because a specific flow is produced only in the required area by the high pressure nozzle, and since processing is possible at an adequately large gap, it is extremely easy to control the gap for stabilizing the flow. Moreover, this processing is accomplished through the electrochemical interaction of the hydroxide ions and workpiece atoms, so there is no loss of workpiece characteristics, and since only hydrogen ions, hydroxide ions, and water molecules are present in ultra-pure water, and no metal ions or other such impurities are present, unlike with the aqueous solutions used in electrolytic processing and the like, if impurities from the outside are completely blocked off, then the processing can be performed in a perfectly clean atmosphere. A considerable reduction in cost is also possible since only ultra-pure water is used.
The second object of the present invention is to provide a method for washing with hydroxide ions in ultra-pure water, with which a shear flow of at least a specific velocity gradient having a controlled range and distribution is generated along the surface of a material to be washed, and the hydroxide ion density is increased on the washing surface of the washed material, or an electrical field is utilized to promote the stripping of ionic impurity metals, the result of which is that the above-mentioned problems are all solved, fine impurity metals can be completely removed, which was very difficult with ultrasonic washing in water, for example, and furthermore the removed impurity metals are prevented from re-adhering to the surface of the washed material, allowing the washing to be performed at greater efficiency.
In view of this, in order to achieve the stated second object, the second invention is a method for washing with hydroxide ions in ultra-pure water, comprising the steps of disposing a workpiece and a high pressure nozzle a specific distance apart from each other inside a washing tank containing only ultra-pure water, providing an ion exchange material or catalyst material that increases the amount of hydroxide ions between the processing surface of said workpiece and the distal end of the high pressure nozzle facing said surface, generating a high-speed shear flow of ultra-pure water sprayed from the high pressure nozzle near the surface of the workpiece, supplying hydroxide ions produced from the ultra-pure water to the workpiece surface, stripping from the workpiece surface any fine impurity metals adhering to the workpiece surface by means of the high-speed shear flow and a chemical elution reaction with hydroxide ions, and using the high-speed shear flow to prevent the impurity metals thus removed from re-adhering to the workpiece surface.
The second invention is also a method for washing with hydroxide ions in ultra-pure water, comprising the steps of disposing a workpiece and a high pressure nozzle a specific distance apart from each other inside a washing tank containing only ultra-pure water, applying a voltage using the high pressure nozzle as the cathode and the workpiece as the anode, generating a high-speed shear flow of ultra-pure water sprayed from the high pressure nozzle near the surface of the workpiece, stripping from the workpiece surface any fine impurity metals adhering to the workpiece surface by means of the high-speed shear flow and a chemical elution reaction with hydroxide ions, and using the high-speed shear flow to prevent the impurity metals thus removed from re-adhering to the workpiece surface.
The second invention is further a method for washing with hydroxide ions in ultra-pure water, comprising the steps of disposing a workpiece and a high pressure nozzle a specific distance apart from each other inside a washing tank containing only ultra-pure water, providing an ion exchange material or catalyst material that increases the amount of hydroxide ions between the processing surface of said workpiece and the distal end of the high pressure nozzle facing said surface, applying a voltage using the high pressure nozzle as the cathode and the workpiece as the anode, generating a high-speed shear flow of ultra-pure water sprayed from the high pressure nozzle near the surface of the workpiece, supplying hydroxide ions produced from the ultra-pure water to the workpiece surface, stripping from the workpiece surface any fine impurity metals adhering to the workpiece surface by means of the high-speed shear flow and a chemical elution reaction with hydroxide ions, and using the high-speed shear flow to prevent the impurity metals thus removed from re-adhering to the workpiece surface.
With these second inventions, if the spray opening in the high pressure nozzle is a round hole, then spot washing can be performed, and if the spray opening in the high pressure nozzle is a slit, line washing can be performed, undulating shape, or cylindrical shape over a large surface area. Also, it is preferable with the washing method of the present invention if a recovery means is provided on the downstream side of the high-speed shear flow generated by the high pressure nozzle, and any process reaction substances removed from the workpiece are recovered, because the removed impurity metals can be prevented from adhering to the workpiece surface.
With a method for washing with hydroxide ions in ultra-pure water as in the second inventions described above, hydroxide ions are produced by an ion exchange material, and these are efficiently supplied to the washed material surface by an electrical field and the flow of ultra-pure water sprayed from the high pressure nozzle, the result of which is that the density of hydroxide ions is higher near the washed material surface. Also, any reaction substances produced in the reaction between the hydroxide ions and the impurity metals adhering to the washed material surface are immediately removed from this surface by a high-speed shear flow of ultra-pure water, and re-adhesion can be prevented, so complete washing can be performed in a perfectly clean atmosphere. This effect can be utilized in the washing away of metal contaminants on a silicon wafer, which is particularly important in the field of semiconductors. Also, the washing apparatus can be more compact because a specific flow is produced only in the required area by the high pressure nozzle, and since washing is possible at an adequately large gap, it is extremely easy to control the gap for stabilizing the flow. Moreover, this washing is accomplished through the electrochemical interaction of the hydroxide ions and impurity metals, so there is no loss of the characteristics of the washed material.